Some papers refer to Omega_0 without defining it. (Example: The first footnote on page 2 in http://arxiv.org/PS_cache/astro-ph/pdf/9908/9908159v3.pdf.) What's the normal assumption in this case? Omega_0 = Omega_m (Omega_m as in Omega_m + Omega_Lambda = 1 in LambdaCDM) fits the context, but seems strange in the above paper because the authors use the notation Omega_m elsewhere in the paper. Is Omega_0 = Omega_m? A related question: That paper refers to the "universal density" rho_u. I haven't heard that term before. rho_u = Omega_m * rho_critical fits the context. Is "universal density" = Omega_m * rho_critical?